A new battery can't fix a bad alternator
If your alternator isn't charging, the car runs on battery power until the battery is drained, then it quits, often in traffic. Put a new battery in and the same thing happens, just later. That's why every battery replacement starts with an alternator test, and why it's worth doing on its own if your car has been dying while you drive.
Car-Battery.xyz techs do batteries, so the alternator test is about getting the diagnosis right. If the alternator is the problem, you'll know before you spend anything, and the repair itself is handled by a mechanic through Mobile Auto Repair.
+ 01 The test
What gets measured
- Charging voltage at idle, usually about 13.7 to 14.7 volts on a healthy system
- Voltage with headlights, blower and rear defrost on, to see if output holds under load
- Ripple voltage, which reveals failing diodes that leak AC current and drain the battery
- Belt condition and tension, since a slipping belt can't spin the alternator properly
- Cable and ground connections that can make a good alternator look bad
Signs it might be the alternator
- Dies while driving
- The car starts with a jump but stalls a few miles later.
- Battery light
- The battery-shaped warning light comes on while driving.
- Dim or flickering
- Lights brighten and dim with engine speed.
- Whine or squeal
- A whine that rises with revs, or a squealing belt.
- Repeat dead batteries
- A new battery that goes flat within weeks.
- Burning smell
- A hot electrical or rubber smell from the engine bay.
Questions about alternator test
Can a bad battery damage the alternator?
It can add strain. A battery with a shorted cell makes the alternator work at full output constantly, which shortens its life. That's another reason to test both.
Do you replace alternators?
Car-Battery.xyz is batteries only. If the test shows the alternator has failed, the tech tells you, and Mobile Auto Repair has mobile mechanics who replace alternators on-site.
Will a jump start tell me if it's the alternator?
Partly. If the car dies again soon after a jump, the alternator is a strong suspect. A proper test confirms it.